A low-phase-noise LC-VCO with an enhanced-Q varactor for use in a high-sensitivity GNSS receiver

Abstract: An LC-VCO with an enhanced quality factor (Q) varactor for use in a high-sensitivity GNSS receiver is presented. An enhanced A-MOS varactor is composed of two accumulation-mode MOS (A-MOS) varactors and two bias voltages, which show the improved Q and linearization capacitance-voltage (C-V) curve. The VCO gain (K_VCO) is compensated by a digital switched varactors array (DSVA) over entire sub-bands. Based on the characteristics of an A-MOS, the varactor in a DSVA is a high Q fixed capacitor as it is switched off, and a moderate Q tuning varactor when it is switched on, which keeps the maximal Q for the LC-tank. The proposed circuit is fabricated in a 0.18 μm 1P6M CMOS process. The measured phase noise is better than -122 dBc/Hz at a 1 MHz offset while the measured tuning range is 58.2% and the variation of K_VCO is close to ±21% over the whole of the sub-bands and the effective range of the control voltage. The proposed VCO dissipates less than 5.4 mW over the whole operating range from a 1.8 V supply.

Key words: A-MOS, voltage controlled oscillator, quality factor, VCO gain compensation, CMOS, CMOS